For tens of millions of years, Earth’s climate may have been stabilized by a hidden feedback linking sea level, ocean oxygen, phosphate, and carbon burial. New geological evidence suggests that this system helped control how much carbon dioxide remained in the atmosphere. Credit: Shutterstock

Researchers found that a narrow range of ocean conditions maximized carbon burial for millions of years at a time.

For more than 100 million years, Earth has remained within a temperature range suitable for life, as though the planet were regulated by a natural thermostat. Exactly how that system worked has remained difficult to explain. New research points to a missing connection involving sea level, phosphate, and the amount of carbon locked away in ocean sediments.

Temperature altered polar ice sheets and sea level, which changed how much phosphate was available to marine life. That nutrient supply then influenced carbon burial, atmospheric carbon dioxide, and ultimately whether the planet became warmer or cooler.

A study coauthored by Zunli Lu, professor of Earth and environmental sciences in the University’s College of Arts and Sciences, examined how changes in sea level and dissolved oxygen shaped ocean phosphate availability and the accumulation of atmospheric carbon dioxide over the past 60 million years. The findings were published in Proceedings of the National Academy of Sciences.

Zunli Lu. Credit: Syracuse University

“We know that atmospheric carbon dioxide decreased substantially as Earth cooled over the last 60 million years, but we have had remarkably little understanding of where that carbon ended up,” says lead author Ros Rickaby, professor of Earth sciences at the University of Oxford, in a department news article. “Our results suggest that enhanced burial of organic carbon in marine sediments played a much more important role than was previously appreciated.”

Phosphate connects sea level and climate

At the center of the process is phosphate, a form of phosphorus that marine organisms need to grow. The researchers describe it as a previously “invisible” part of the climate puzzle.

When sea levels were high, wide continental shelves captured phosphate in shallow seafloor sediments before it could reach the open ocean. With less of the nutrient available, marine productivity fell, and less organic carbon sank and became buried on the seafloor. Ocean waters remained rich in oxygen, while more carbon dioxide accumulated in the atmosphere.

Falling sea levels produced the opposite pattern. As continental shelves contracted, more phosphate entered the water, stimulating the growth of marine organisms. When this organic material sank and decomposed, the process used up dissolved oxygen and created expanding zones of oxygen-poor water.

AI-generated graphic describing the findings. Credit: Syracuse University

Once those zones reached organic-rich sediments on continental shelves, they initiated a reinforcing cycle. Low oxygen conditions released additional phosphate from the sediments, supporting more marine growth and increasing the burial of organic carbon. As more carbon was trapped beneath the seafloor, less carbon dioxide remained in the atmosphere.

“Our co-author, Christian Bjerrum, studied the connection among sea level, ocean oxygen and phosphate with a computer model two decades ago,” Lu says. “We finally pieced together the geologic records necessary to test this hypothesis.”

A narrow sea level range maximized burial

The strongest carbon burial occurred within a relatively limited window, when sea level stood about 10 to 40 meters above its present height. Within this sea level “sweet spot,” oxygen minimum zones aligned with organic-rich shelf sediments, allowing the feedback to continue for millions of years.

The researchers tested this pattern against geological evidence covering 60 million years. Their records included carbon isotopes, phosphorus accumulation in deep-sea sediments, and a newly developed iodine-to-calcium proxy for estimating oxygen levels in ancient oceans.

Lu’s laboratory performed the iodine-to-calcium measurements. The method examines the preserved chemistry of foraminifera, microscopic marine organisms whose remains accumulate in seafloor sediments, to determine how much oxygen was present in ancient water. The samples were measured with a mass spectrometer at Syracuse University through funding from the National Science Foundation.

High seas weakened Earth’s carbon sink

The Eocene epoch, spanning approximately 56 to 34 million years ago, provides a clear example of what happened when the carbon burial feedback stopped operating effectively. Sea levels were extremely high, flooding continental shelves and trapping phosphate in shallow sediments. The open ocean remained highly oxygenated, while the mechanism that promoted organic carbon burial was largely inactive.

With less carbon being removed and stored in marine sediments, carbon dioxide accumulated in the atmosphere and Earth remained warm.

The study proposes that the range of conditions favorable for carbon burial became narrower over geological time as oxygen minimum zones shifted into deeper water. This gradual change helped stabilize atmospheric oxygen and carbon dioxide. Fluctuations between ocean carbon burial and atmospheric accumulation became less pronounced, increasing the resilience of Earth’s climate system.

Ancient oxygen records sharpen climate history

The results extend earlier work from Lu’s laboratory using the iodine-to-calcium proxy to reconstruct the oxygen content of ancient oceans.

A study published in Nature Geoscience in January used the same method to show that tropical oceans during the Proterozoic Eon contained abundant oxygen, the opposite of the modern pattern. That research also identified a planetary tipping point hundreds of millions of years ago that reversed the distribution of oxygen in the oceans.

Reference: “Shelf-invading low-oxygen waters control Cenozoic organic carbon burial rates” by Rosalind E. M. Rickaby, Thomas J. Wood, Zunli Lu and Christian J. Bjerrum, 22 June 2026, Proceedings of the National Academy of Sciences.
DOI: 10.1073/pnas.2526409123

The project received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (SCOOBI project, grant agreement no. 101019146; REMR) and from the Natural Environment Research Council (NERC; PUCCA project, award NE/V011049/1; REMR).

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